1. Field of the Invention
The present invention relates to alignment marks used for alignment in manufacturing a semiconductor device or the like, and relates to a method of forming the alignment marks.
2. Description of Related Art
Alignment of a photomask is carried out by the use of alignment marks formed on a substrate where circuit components or elements are formed, when a device such as a semiconductor device or a liquid crystal display is manufactured. The elements are formed on the substrate by carrying out the mask alignment in a predetermined processing step among various processes such as layer formation and impurity doping. A manufacturing process of a semiconductor integrated circuit will be described as an example hereinbelow.
Conventionally, alignment marks are formed in an area, for example, a scribe area where the circuit elements are not formed in patterning process of a circuit pattern on a wafer of a silicon (Si) substrate 1 or the like. The process will be described with reference to the sectional view of FIG. 1. An alignment mark 2 which has differences in surface level or a step-like form is formed at a first alignment mark forming area. The alignment mark 2 is formed by using, for example, a conventional photolithography technique. More specifically, a photoresist layer is applied on the silicon substrate 1. The photoresist layer is exposed using a photomask and is subject to patterning process. An etching process is, then, performed on the substrate 1 to form a predetermined mark-form.
A first upper layer 3 is, then, formed on the alignment mark 2. A second alignment mark 4 is then formed at a position different from the position of the alignment mark 2 by etching the first upper layer 3 through the similar processing steps as mentioned above. An alignment mark is sequentially formed on each deposited layer by performing the above-mentioned steps repeatedly.
A new alignment mark, however, needs to be formed at a place where no alignment mark is formed in the conventional alignment mark forming method. Therefore, an area for arranging alignment marks becomes insufficient as the number of steps of the wafer processing increases. Additionally, a problem occurs where the superposition accuracy decreases as separation increases between the position of an alignment mark used for exposure processing and the position of an alignment mark used for measurement of the superposition accuracy. The alignment marks used for the wafer processing, therefore, need to be formed with high accuracy even in a case in which many process steps are performed.
The present invention has been made in view of the foregoing, and an object of the present invention is to provide alignment marks and a method of forming the alignment marks in which the area for the alignment marks can be reduced and alignment can be performed with high accuracy.
According to the present invention, there is provided a method of forming alignment marks that can be detected by alignment light to form elements on a substrate, which comprises the steps of forming a first alignment mark in an alignment mark forming area on the substrate, forming an opaque layer that is opaque to the alignment light above the alignment mark forming area where the first alignment mark is formed, substantially flattening a surface of the opaque layer, and forming a second alignment mark on a side where the alignment light is incident with respect to the flattened opaque layer.
According to the present invention, there is provided a method of forming alignment marks that can be detected by alignment light to form elements on a substrate, which comprises the steps of forming a first alignment mark in an alignment mark forming area on the substrate, forming at least one layer on the alignment mark forming area where the first alignment mark is formed, substantially flattening a surface of the at least one layer, and forming a second alignment mark on the flattened surface of the at least one layer, wherein the second alignment mark has a shape to intercept reflected alignment light from the first alignment mark.
According to the present invention, there is provided alignment marks that can be detected by alignment light to form elements on a substrate, which comprises a first alignment mark formed in an alignment mark forming area on the substrate, an opaque layer, which is opaque to the alignment light, formed above the first alignment mark, a surface of the opaque layer being substantially flattened, and a second alignment mark formed on a side where the alignment light is incident with respect to the flattened opaque layer.
According to the present invention, there is provided alignment marks that can be detected by alignment light to form elements on a substrate, which comprises a first alignment mark formed in an alignment mark forming area on the substrate, at least one layer formed above the first alignment mark, a surface of the at least one layer being substantially flattened, and a second alignment mark formed on the flattened surface of the at least one layer, wherein the second alignment mark has a shape to intercept reflected alignment light from the first alignment mark.